1. Field of the Invention
The present invention relates to specific mixtures of n-propylethoxysiloxanes, to a process for preparing them, and to their use.
2. Description of the Background
Alkyl-functional silanes and siloxanes are employed, for example, in the hydrophobicization of absorbent mineral materials.
EP 0 814 110 A1 describes the preparation of catenated and/or cyclic C3-C18 alkylalkoxysiloxanes by a two-stage process starting from the corresponding alkyl chlorosilane, which is first esterified to the alkylalkoxysilane and then, secondly, the ester product undergoes controlled hydrolysis and condensation to give the desired alkylalkoxysiloxane product. In this process, more than 1 mol of water per mole of Si is employed for the controlled hydrolysis and condensation. Alkylalkoxysiloxanes obtained in this manner are generally in the form of a mixture of oligomers. Besides alkyl and alkoxy groups, the alkylalkoxysiloxanes may also carry hydroxyl groups, if alcohol is eliminated from the siloxane by hydrolysis. Otherwise, siloxanes, as disclosed in EP 0 814 110 A1, possess a degree of oligomerization, n ranging from 2 to 20, i.e., they have 2 to 20 Si units per molecule, possess a viscosity of up to 100 mPa s, and are used, inter alia, for the hydrophobicization of mineral surfaces.
High-viscosity agents for hydrophobicizing mineral surfaces generally lead to poor penetration behavior of the active substance into the substrate, especially in the case of very compact substrates, such as concrete with a low w/c ratio. The w/c ratio indicates the ratio of water and cement; i.e., the greater the proportion of water, the greater the subsequent amount of pores in the concrete. Furthermore, active substances having a relatively high degree of oligomerization may result in the discoloration of the substrate surface, this discoloration being caused by the failure of higher oligomers to penetrate the substrate, or else the substrate surface is observed at least to have an unwanted sheen or an oily appearance.
EP 0 579 453 A2 teaches a process for preparing alkylalkoxysiloxanes, especially an isobutyltrimethoxysilane-based system, in which from 0.1 to 0.6 mol of water per mole of alkoxysilane is used for controlled hydrolysis. In this case, the products are alcohol-containing mixtures which include a high proportion of silane monomer, i.e., reactant, as a result of which the product on application has a high volatile monomer content and, furthermore, possesses a low flash point. Products of a low flash point are subject to strict transport and safety-at-work regulations in a number of countries, including the EU, Japan, and the USA.
J. Organometallic Chem. 489 (1995) teaches the hydrolysis of i-propyl- and, respectively, i- and n-butyltrimethoxysilane, in which a catalyst, such as dibutyltin laurate, and a solvent, such as tetrahydrofuran, are used.
WO 92/06101 discloses solvent-free organoalkoxysiloxanes having from 2 to 9 Si units for the water repellency treatment of mineral building materials, the repellency properties of the siloxane being enhanced if desired by the addition of organic fluorine compounds. The list of organic groups in the organoalkoxysiloxane includes C1 to C30 alkyl/cycloalkyl/arylalkyl/alkaryl or mixtures thereof, and also olefinic organic groups and organic groups substituted by heteroatoms and/or fluorine atoms. Particular emphasis is placed on organoalkoxysiloxanes with C4-8-alkyl groups and possess a degree of oligomerization ranging from 2 to 4, with 1,3-di-n-octyl-1,1,3,3-tetraethoxy- and -methoxydisiloxane being highlighted in particular. It is complicated and expensive to conduct controlled preparation of such a disiloxane.
U.S. Pat. No. 5,543,173 discloses not only aminoalkyl- and diaminoalkyl-functional polysiloxanols, but also methyl-, ethyl-, 1-propyl- and n-butyl-functional, especially octyl-functional, polysiloxanols. These compounds are prepared by controlled hydrolysis of the corresponding alkoxysilanes, the product including a considerable fraction of alcohol produced by hydrolysis and significant amounts of solvent, such as toluene or methyl isobutyl ketone.
For the hydrophobicization of concrete, EP 0 101 816 B1 discloses the use of solvent-free C3 to C8 alkylalkoxysilanes, i.e., monomeric alkylalkoxysilanes.
Monomeric alkylalkoxysilanes, such as n-propyltriethoxysilane (PTEO), isobutyltriethoxysilane (IBTEO) and octyltriethoxysilane (OCTEO), for example, although they possess a comparatively good penetration behavior, are nevertheless hampered by the drawback that on the basis of their own vapor pressure, they may evaporate, leading to material losses and environmental pollution (VOCs—volatile organic compounds).
It is also known to apply C3 to C8 alkylalkoxysilanes in solution in an alcohol or in other evaporable or volatile solvents. Here again, marked evaporation losses are found. Furthermore, PTEO, IBTEO, and the solutions in highly volatile solvents possess a low flash point.
Furthermore, monomeric PTEO gives emulsions having a flash point below 55° C., and is therefore classified as a flammable liquid requiring appropriately complex storage, transport, and handling.
In order to lessen these drawbacks, particular use has been made in the past of alkoxysilanes of high molecular weight, i.e., alkylalkoxysilanes with an alkyl chain having a large number of carbon atoms, an example being octyltriethoxysilane. OCTEO is generally prepared by hydrosilylating octene with trichlorosilane and then esterifying the product, which is very complex to do. The product in this case carries a high price. n-Propyltrichlorosilane (PTS) is obtained as a byproduct in industrial processes. However, the quantity available on the market exceeds the demand for PTS and PTEO.